MINOR: [C++] Fix StringFormatter type error in localfs_benchmark (#13932)
MINOR: [C++] Fix StringFormatter type error in localfs_benchmark (#13932)

since size_t is passed as the first argument to the StringFormatter, it
needs to be templated with Int64Type instead of Int32Type

Lead-authored-by: Aldrin M <75c199c3a31b60f91567634037d820c919fad644@pm.me>
Co-authored-by: Aldrin Montana <75c199c3a31b60f91567634037d820c919fad644@pm.me>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
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